Voltage-controlled oscillators (VCO) are critical components in wireless communication systems. As an example, signals of different channels may be selected by changing the output frequency of the VCO. Recently, due to the increasing demand for data transmission, it is important to effectively improve on the operating frequency and the adjustable frequency range.
Referring to FIG. 1, a conventional VCO includes two inductors (Lg), two varactors (Cd), a cross-coupled pair of transistors (M1, M2), and a current source 11.
A common node of the inductors (Lg) is coupled to a voltage source (VDD), the varactors (Cd) are respectively coupled to the inductors (Lg) and have a common node coupled to a bias voltage source (Vtune) the drain terminal and the gate terminal of each of the transistors (M1, M2) are respectively coupled to the varactors (Cd), and the current source 11 is coupled between a common source node of the transistors (M1, M2) and ground.
In application, the voltage provided by the bias voltage source (Vtune) may be adjusted to change capacitances of the varactors (Cd), thereby adjusting an output frequency of the VCO.
However, the operating frequency and the adjustable frequency range of the conventional VCO may be limited due to effective parasitic capacitance of the transistors (M1, M2). In addition, the voltage provided by the voltage source (VDD) has to be raised for enabling the transistors (M1, M2) to operate in the saturation region while maintaining sufficient output voltage swing, resulting in higher power consumption.